Vehicle systems may include various vacuum consumption devices that are actuated using vacuum. These may include, for example, a brake booster. Vacuum used by these devices may be provided by a dedicated vacuum pump. In still other embodiments, one or more aspirators (alternatively referred to as ejectors, venturi pumps, jet pumps, and eductors) may be coupled in the engine system that may harness engine airflow and use it to generate vacuum. For example, when incorporated in an engine intake system, aspirators may generate vacuum using energy that would otherwise be lost to throttling.
Since aspirators are passive devices, they provide low-cost vacuum generation when utilized in engine systems. An amount of vacuum generated at an aspirator can be controlled by controlling the motive airflow rate through the aspirator. While aspirators may generate vacuum at a lower cost and with improved efficiency as compared to electrically-driven or engine-driven vacuum pumps, their use in engine intake systems has traditionally been constrained by both available intake manifold vacuum and maximum throttle bypass flow. Some approaches for addressing this issue involve arranging a valve in series with an aspirator, or incorporating a valve into the structure of an aspirator. Such valves may be referred to as aspirator shut-off valves (ASOVs). An opening amount of the valve is controlled to control the motive airflow rate through the aspirator, and thereby control an amount of vacuum generated at the aspirator, as well as controlling the amount of air flowing past the aspirator into the intake manifold.
However, the use of an ASOV leads to increased component cost and weight. In addition, there may be reliability issues associated with the use of an ASOV. To confirm the proper functionality of the ASOV, intermittent diagnosing of the ASOV is required, adding control complexity. Further still, in engine systems operating with an ASOV, the engine controller is required to be aware of the airflow rate through the ASOV, adding further control burden to the system. The inventors herein have recognized that if the motive mass flow rate through the aspirator were sufficiently reduced, dependence on an ASOV for controlling flow through the aspirator could also be reduced. For example, the ASOV could be eliminated. The inventors have further recognized that motive mass flow rate through the aspirator can vary based on a temperature of the aircharge flowing through the aspirator. Thus in one example, a method for operating an engine is provided, comprising: flowing intake air, heated upon passage through an interstitial space of a double wall exhaust system, through an aspirator coupled to an engine vacuum consumption device to reduce motive mass flow rate at the aspirator as exhaust temperature increases. In this way, heated intake air can be used during idle conditions to reduce a motive mass flow rate through an intake aspirator, while also reducing airflow errors across an intake throttle.
As an example, during an engine cold-start, intake air may be flowed from upstream of an intake throttle, through an interstitial space of an exhaust manifold, and then through an aspirator before being delivered to the intake manifold, downstream of the intake throttle. The aspirator may be un-valved. A position of the throttle may be adjusted based on operating conditions, such as torque demand. The throttle position may additionally be feed-forward adjusted to compensate for aspirator leakage mass flow (that is, air flowing through the aspirator). As such, this may be considered a leak around the throttle valve. At low exhaust temperature conditions, the temperature of air flowing through the aspirator (the motive flow) may be lower, and aspirator motive mass flow rate may be higher. During such conditions, the throttle may be moved to a more closed position to achieve a desired net mass flow rate. As the exhaust temperature increases, the intake air reaching the aspirator via the interstitial spaces may be heated. As the motive flow temperature increases, the motive mass flow rate may decrease. Thus, as the engine warms up, the throttle may be moved to a more open position, all other things being equal. However, as the engine warms up, the idle air mass airflow rate is reduced, resulting in a greater portion of the idle air being warmed serendipitously. By adjusting flow conditions such that these two effects are able to substantially offset each other, the same throttle angle would be able to satisfy both conditions, and a throttle angle can be maintained during airflow control. Herein, the heated, less dense air would serve to give the requisite lower net mass flow needed by a warmed up idling engine. In addition, the throttle position may also be feedback adjusted based on deviations between the expected airflow and the actual airflow following any leakage compensation.
In this way, by heating air delivered to the engine via an aspirator, a motive mass flow rate can be decreased, reducing the mass flow rate into the engine below which the throttle cannot achieve. By enabling the throttle to be held less closed during idling conditions, the error state of allowing too much air mass rate at idle is lessened and the reliance on spark retard for idle speed control is also reduced. In this way, idle speeds can be maintained while drawing vacuum at an aspirator and without reducing fuel economy. In addition, the heated air exhausted into the intake manifold may reduce icing of the air and crankcase plumbing during engine cold starts. By using changes in motive flow temperature to control a motive mass flow rate, the need for a dedicated ASOV for motive flow control is reduced, providing component and cost reduction benefits.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.